I’m not exactly sure where your the problem is, but I just tried inserting a new post and then updating a repeater field and it worked for me.
Here is the test code that I ran
$post = array(
'post_type' => 'post',
'post_title' => 'inserted post test 2',
'post_status' => 'publish',
'post_content' => 'test content',
);
$post_id = wp_insert_post($post);
$value = array(
array(
'field_5c1abd1da3f56' => 'test update field'
)
);
update_field('field_5c1abd11a3f55', $value, $post_id);